The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (Aug. 1988), Vol. 39, No. 5 (Oct. 1988), Vol. 43, No. 4 (Aug. 1992), Vol. 43, No. 6 (Dec. 1992) and Vol. 45, No. 1 (February 1994).
An ink-jet image is formed when a precise pattern of dots is ejected from a drop generating device known as a "printhead" on a printing medium. The typical ink-jet printhead has an array of precisely formed nozzles attached to a thermal ink-jet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) from an ink reservoir. Each chamber has a thin-film resistor, known as a "firing resistor," located opposite the nozzle so ink can collect between the firing resistor and the nozzle. When electric printing pulses heat the thermal ink-jet firing resistor, a small volume of ink adjacent the firing resistor is heated, vaporizing a bubble of ink, and thereby ejecting a drop of ink from the printhead. The droplets strike the printing medium and then dry to form "dots" that, when viewed together, form the printed image. The printhead is held and protected by an outer packaging referred to as a print cartridge. in one aspect of ink-jet ink delivery, the reservoir for storing the ink may be placed in the print cartridge (on-board ink reservoir). The print cartridge has a self-contained reservoir for storing ink and providing appropriate amounts of ink to the printhead during a printing cycle. These are disposable and are replaced when the ink is exhausted. Further, the ink can be stored in the reservoir in different ways. The ink may be stored in a contained medium, such as a permeable foam material (U.S. Pat. No. 4,771,295 (Baker et al)). Alternatively, the ink reservoir can be a free-ink type, having a biased ink bladder or bag as shown in U.S. Pat. No. 5,359,353 (Hunt et al).
One problem in ink-jet printing is that some applications require a large supply of ink. For example, "large format" applications use large size printing media (for example, 22 inch.times.34 inch, 34 inch.times.44). Examples of large format applications include computer aided design (engineering drawings), mapping, graphic arts, and posters. The large format printed image can use a large amount of ink either because of the large printed area needing to be covered with ink or the use of 100 percent filled-in image areas, or both. Therefore, it is desirable to have ink reservoirs that contain a large amount of ink to avoid replacing an empty ink reservoir in the middle of a printing cycle or the frequent changing of the ink reservoir between printing jobs.
However, merely increasing the size of the ink reservoir in an on-board system to hold more ink has not proved to be an acceptable solution. The ink reservoir is supported on the printer carriage and moves with the printhead. Increasing the amount of ink in motion would necessarily require an increase in the size and weight of the structure that supports and moves the carriage back and forth. The increased mass of the carriage would also significantly increase the cost of the printer (for example, larger and more expensive electrical motors).
In response, recently, relatively large ink reservoir systems have developed in which the reservoir is mounted off-board. Examples include U.S. Pat. No. 4,831,389 using a low pressure recirculating pumping system; and U.S. Pat. No. 4,968,998 (Allen) which teaches an ink-jet print cartridge which is refillable at a service station.
In contrast to on-board ink reservoirs, printing systems using off-board ink reservoirs require means for delivering the ink from the off-board ink reservoir to the printhead. Pumps can be used for such delivery, but such pumps have problems associated with their use. For example, the ingredients in the ink can be incompatible with the pump components, and such components as diaphragms and seals can degrade when exposed to the ink solvents for extended time periods.
A second problem in ink-jet ink delivery arises in color printing. Color printing typically uses multiple ink reservoirs, each containing ink of a different hue. Since each ink reservoir must be individually pressurized, multiple pumps can be used. However, the addition of each additional pump increases the cost of the overall printing system. Thus, it would be desirable to use one pump that can provide the necessary pressure for all the ink reservoirs individually.
One other problem in ink-jet technology is that the customers have different purchasing criteria. Some customers, with high ink usage rate, may prefer the lower, "unit price" of a large ink reservoir. Other customers, may prefer a lower, "start-up" price of a smaller ink reservoir. Thus, it would be beneficial for the customers to have a printing system that is adaptable to ink reservoirs with different sizes. In addition, the manufacturer also benefits when the size of the ink reservoir is not a limiting factor in the design of the printer or the ink delivery system.
It will be apparent from the foregoing that although there are many processes and apparatus for providing ink to the printhead from an off-board ink reservoir, there is still a need for an approach that provides low material cost and adaptability to various sizes of ink reservoirs, and no requirement for additional pressurization devices.